Objects imaged by conventional imaging subsystems are sharply in focus over a limited distance known as the depth of field, which is inversely proportional to the square of the imaging system's numerical aperture for diffraction-limited imaging. Present-day cameras have mechanical focusing means, including automatic systems, to provide high quality images of particular scenes at various object distances. Even with these means, it is difficult to photograph objects clearly that span a large range of such distances. Cameras with a larger depth of focus will clearly provide superior photographs.
Digital processing of image data on a pixel-by-pixel basis has afforded more opportunity for improving and correcting optically imaged scenes. Some of these improvements have related to increasing the depth of field. For example, digital processing has been used to combine images of the same scene taken at different depths of focus to produce a composite image having an extended depth of field. The multiple images take time to collect, are difficult to process, and are generally unsatisfactory for scenes subject to change.
Amplitude attenuation filters have also been used to extend the depth of field. Typically, the attenuation filters are located in the aperture of the imaging systems, leaving inner radii clear but attenuating the outer annulus. However, the filter introduces large amount of light loss, which limits its applications.
More promising attempts have been made that deliberately blur an intermediate image in a systematic way so that at least some information about the imaged object is retained through a range of focus positions and a non-ideal impulse response function remains substantially invariant over the defocus range. Digital processing, which effectively deconvolutes the point spread function, restores the image to a more recognizable likeness of the object through an extended depth of field.
One such example locates a cubic phase mask within the aperture of the imaging system to generate a distance invariant transfer function, thereafter. Digital processing removes the blur. Although significant improvement in the depth of field is achieved, the cubic phase mask is not rotationally symmetric and has proven to be expensive and difficult to fabricate.
Another such example similarly locates a circularly symmetric, logarithmic asphere lens to extend the depth-of-field, which is more economical to manufacture. However, for the log-asphere lens, the impulse response is not perfectly uniform over the full range of operation, and as a result, some degradation is experienced in the image quality of the recovered image.
Reconstruction algorithms for removing the blur of such intermediate images are subject to problems relating to the quality and efficiency of their results. Nonlinear processing algorithms can suffer from slow convergence or stagnation and produce images with reduced contrast at high spatial frequencies.